Automatic emission control circuit



June 28, 1955 I M. BEVIS 2,712,075

AUTOMATIC EMISSION CONTROL CIRCUIT Filed Jan. 26, 1945 INVENTOR. murraqfie vzlr United States Patent" AUTOMATIC EMISSHON CONTROL CIRCUIT MurrayBevis, Oak Ridge, Tenn, assignor, by mesne assignments, to the UnitedStates of America as represented by the United States Atomic EnergyCommission Application January 26, 1945, Serial No. 574,664

3 Claims. (Cl. 250-419) This invention relates to the automatic controlof electron discharge devices and it is particularly concerned withautomatic control of the emissivity of a cathode. More particularlystill the invention is concerned with arrangements embodying a cathodeand an anode wherein an arc is struck between the cathode and the anodeand this may be in an atmosphere consisting of the vapor of a solidmaterial. The invention finds particular application in isotopeseparating apparatus wherein there is an ion source comprised of meansfor vaporizing a solid material, the isotopes of which it is desired toseparate, and ionizing means consisting of a cathode and an anodebetween which an arc is struck in an atmosphere of the vapor of thesolid material. This type of apparatus is described in detail in theprior application of Ernest 0. Lawrence, Ser. No. 557,784 filed October9, 194.4. In this type of apparatus the ion source is within a spaceevacuated to a relatively high degree of vacuum and a relatively intensemagnetic field is provided which is in the same direction, that is, itis parallel to the are which is struck between the cathode and anode or,in other words, thearc which is struck follows the direction of themagnetic field. In this apparatus, when the vacuum is at the desiredvalue and also the magnetic field, and the proper quantity of materialis being vaporized and the cathode is emitting, an arc will be struckbetween the cathode and anode depending upon all of these variousfactors and the arc may be said to have struck when its value is oneampere, for example. A higher temperature of the cathode is required tocause the arc to strike than to sustain it after it has struck,becauseof low initial emissivity. Q

A primary object of my invention is to provide an automatic timingarrangement which periodically raises the temperature or emission of thecathode to a relatively high value for relatively shortintervals untilthe are is struck and then sets the temperature down to a steadyintermediate value. During the period preceding striking of the are, orattempting to strike the arc, and during which the other conditionsreferred to above are being adjusted and stabilized, the temperature ofthe cathode may be set at a relatively lower standby value. The purposeof the arrangement is to prolong the life of the cathode, which may be afilamentary cathode, by not operating it continuously at hightemperature when no arc has been struck.

Another object of the invention is to provide an auto maticcontrolarrangement for periodically'raising the emission of a cathode toa relatively high value for short intervals and then reducing theemission to a lower steady value in response to a relay mechanismcontrolled by an arc struck between the cathode and anode.

Other objects of my invention and numerous of its figure is a wiringdiagram of the automatic control ar rangement of my invention as appliedto an ion source of the type referred to above.

Patented June 28, 1955 ice Referring to the drawing, numeral 10designates generally an ion source of the type described in detail inthe application referred to above. The ion source may be a part of theisotope separating apparatus as described in the earlier application andit is disposed within an evacuated envelope or space indicated by thebroken lines 11. The ion source comprises a chamber, as indicated at 12,having an enlarged lower portion and a smaller upper portion connectedby a converging throat asshown, the upper portion having an opening 13in the form of a slit. A charge of solid material to be ionized isplaced within the chamber 12 and it is vaporized by electric heatingelements designated by numeral 14, the supply of heat thereto beingcontrolled by any of various suitable types of automatic control system.Disposed adjacent one end of the slit 13 is a filamentary cathode 15 anddisposed adjacent the other end of the slit and attached to the chamber12 is an anode or anode plate 16. When the ion source is a part of theappara tus as described in the earlier application referred to, amagnetic field is provided which is parallel to the slit 13 as indicatedby the arrow adjacent thereto. Vapor of the charge material in thechamber 12 passes up past baffles 17 through the narrow portion of thechamber and upwardly through slit 13. In operation a stream of electronsis emitted by the cathode 15 and they are collimated by the magneticfield and form an arc between the cathode 15 and anode 16.

My invention is particularly concerned with automatic control of theemission of the cathode 15. The cathode 15 is supplied with power from atransformer 20 having a primary winding 21 and a secondary winding 22,the flow of power to the cathode being controlled by a saturable reactor23 which is in circuit therewith as shown. The saturable reactor is ofconventional design of this type of device, it consists of alternatingcurrent windings 24 and 25 and a D. C. winding 26, the energization ofwhich controls the amount of energy which the saturable reactor feeds tothe cathode.

A suitable D. C. potential is applied between the cathode 15 and theanode 16, that is, between the cathode 15 and the chamber 12 to whichthe anode 16 is attached, by leads 31 and 32. Lead 32 connects to thechamber 12 through a voltage divider 33 and the other lead '31 connectsto one of the leads to the cathode 15 as shown. The voltage divider 33controls the energization of a relay'34 having a winding 35 and a switch36 which closes when the relay is energized and this occurs when thereis a predetermined current in the voltage divider 33 so as to cause apredetermined voltage to be impressed on the winding 35.

My invention is particularly concerned with the control of theenergization of winding 26 of saturable reactor 23 and with automaticvariation of the energization of this winding in response to actuationof the relay 34.

Numeral 40 designates generally an electronic timing arrangementcomprising a pair of similar tubes 41 and 42, the filaments of which aresupplied with power from the secondary winding 43 of a transformer 44having a primary winding 45. The primary winding 45 is supplied withpower by leads 46 and 47 which are connected to a suitable source ofpower under control of a manual switch 48 and a relay 79 which willpresently be described. The secondary winding 43 supplies power to thefilaments of the tubes 41 and 42 which have indirectly heated cathodes,the connection being diagrammatically designated by the characters X-X.With reference to the timing device 40, the plate of tube 41 connects tothe grid of .tube 42 through a condenser 50, and the plate of tube 42connects to the grid of tube 41 through a condenser 51. In the platecircuit of tube 41 is a resistance having a condenser- 56 connectedacross it, and in the plate circuit of tube 42 is a resistance 57 inseries with a winding 58 of a relay 59 having a switch 60 which isnormally open and which closes when the relay 59 is energized. Acondenser 61 is connected across the resistance 57 and winding 58.Numeral 65 designates a series of resistances associated with anadjustable tap 66, any one of which can be connected so that thecondenser 50 can charge therethrough when tube 41 is conducting. Numeraldesighates a similar set of resistances having an adjustable tap 71associated therewith whereby any one of the resistances can be connectedso that the condenser 51 can charge therethrough when the tube 42 isconducting.

In the operation of the timing device 40, when the transformer 44 isenergized and when tube 41 is conducting, there will be a voltage dropacross resistance 55 so that its lower end will be at a negativepotential which will make the grid of tube 42 negative relative to itscathode by reason of the connection through condenser 50. Tube 42 willthus be nonconducting, condenser 50 will charge through one of theresistances 65, until the grid of tube 42 is brought to cathodepotential, the time required depending upon the value of the resistancewhich is placed in circuit. When tube 42 becomes conducting, the currentflow through resistance 57 and winding 58 will cause the upper end ofresistance 57 to be at a negative potential which will cause the grid oftube 41 to also be negative rendering this tube non-conducting. Thecurrent flow through winding 58 will energize relay 59 causing closureof switch 60. Condenser 51 will now charge through one of theresistances 70 until the grid of tube 41 is brought to cathode potentialrendering this tube conducting, the time required for it to charge beingdependent upon the resistance connected in circuit with the condenser.The tubes 41 and 4-2 will alternately be conducting in the mannerdescribed above, relay 59 becoming energized each time that tube 42 isconducting, the on and off periods of the relay 59 being adjustable byadjusting the taps 66 and 71, tap 71 serving to adjust the on period ofthe relay and tap 66 serving to adjust the ofl. period. In my particularsystem the on period may be 60 seconds, for example, and the off periodmay be 500 seconds as will presently be referred to again.

Relay 59 controls another relay 75 having a winding 76 controllingswitches 77 and 78 which are normally open but which close when therelay 75 is energized.

Numeral 79 designates another relay having a winding 80 which controlsswitches 81 and 82, switch 81 normally being closed and opening when therelay is energized and switch 82 normally being open but closing whenthe relay is energized. Relay 79 is controlled by the switch 36 of relay34, the relay 79 becoming energized when the energization of relay 34causes the closing of switch 36.

Switch 77 of relay 75 controls a red signal light 85 supplied with powerfrom the secondary 43 of transformer 44. as indicated by the charactersX-X. Numeral 88 designates a green signal light which is continuouslyconnected across this same source of power. Switch 78 of relay '75 isarranged so that when it is closed it shunts two variable resistances90and 91 which are normally in circuit with the D. C. winding 26 ofsaturable reactor 23, this winding normally being supplied with powerfrom a D. C. source by way of leads 92 and 93. Either of the resistances90 and 91 may be shunted by manual switches 95 and 96 respectively,associated therewith. The switch 82 of relay 79, when closed, shuntsresistance 90 out of the circuit of winding 26 but not resistance 91.

Switch 81 of relay 79 controls the supply of power to the transformer44, that is, it controls the operation of the electronic timer. 7

In the standby operation of the system as described, the supply of powerto the D. C. winding 26 of saturable reactor 23 is through both of theresistances 90 and 91, both of which are variable, so that the emissionof cathode 15 is at a relatively low standby value. The D. C. winding ofthe saturable reactor 23, of course, controls v the degree of saturationthereof and therefore the amount of power which is fed to the cathode 15from transformer 20. The standby value of the emission can, of course,be varied by adjusting either of the resistances 90 or 91.

When the conditions associated with the ion source 10 are such that itis desired to attempt to strike the are between the cathode 15 and anode16, the manual switch 48 is closed energizing the transformer 44. theswitch 81 being closed at this time. This starts the timer 40 in themanner described above causing switch 60 of relay 69 to be periodicallyclosed and to cause the relay 75 to similarly periodically close itsswitches. Thus the resistances 9t) and 91 are periodically shunted outof the circuit of D. C. winding 26 so that the power supplied to thecathode 15 is considerably increased for intervals, that is, theintervals during which relays 59 and 75 are energized. The increasedpower supplied to the cathode 15 raises its emission to a relativelyhigh value, and when the conditions associated with the ion source 10are proper, an arc will be struck between the cathode 15 and the anode16. When the arc has struck, and it may be said to have struck when ithas a value of one ampere for example, it is no longer necessary tomaintain the emission of cathode 15 at as high a value as was necessaryto cause the arc to strike. When the arc strikes, there will, of course,be current in the arc circuit, that is, the circuit having in it the D.C. supply 3132 and the voltage divider 33. Thus, the voltage will beincreased across the winding 35 of relay 34 energizing this relay andclosing switch 36. Closure of switch 36 will energize winding 80 ofrelay 79, which is connected across the power supply 46--47, andenergization of this relay will open switch 81 and close switch 82.Opening of switch 81 will de-energize transformer 44 cutting off thepower supply to and stopping the operation of the electronic timer 40.Thus, periodic operation of relays 59 and 75 will cease. Closure ofswitch 82 will shunt the resistance 90 out of the circuit. of the D. C.winding 26 of saturable reactor 23 but not the variable resistance 91.Thus the power supply to the D. C. winding 26 will now be at anintermediate value which is higher than the standby value describedbut'lower than the value it is at when relay 75 is energized and bothresistances 90 and 91 are shunted. The power supply to the D. C. winding26 will now be such that saturable reactor 23 will feed enough power tothe cathode 15 to maintain its emission at a value high enough tosustain the arc between cathode 15 and anode 16. This emission can beadjusted by adjusting the variable resistance 91 which is still incircuit with D. C. winding 26 when relay 79 is energized.

The emission of cathode 15 can be raised to its maximum value at anytime, of course, simply by closing both of the manual switches 95 and 96or by closing only the manual switch 96 while relay 79 is energized.

From the foregoing those skilled in the art will observe that I haveprovided an efficient automatic system for preserving and lengtheningthe life of a cathode by controlling its emission in accordance withrequirements necessary for operation. The emission is automaticallyraised at intervals until conditions are such as to cause the desiredarc to strike. The emission is then automatically lowered to a value asnecessary to sustain the arc. The arrangement provides for all of thenecessary adjustments for varying the periodicity and duration of theintervals of high emission and for adjusting the values of the emissionat the highest stage and in the intermediate and standby stages.

The foregoing disclosure is representative of the preferred form of myinvention and it is intended that it be interpreted in an illustrativerather than a limiting sense, and that the scope of the invention bedetermined only in accordance with the claims appended hereto.

I claim:

1. In a control apparatus in combination, means forming a cathode, meanscomprising an anode, and said cathode and anode being adapted to have anarc struck therebetween, automatic control means for controlling theemission of said cathode comprising electric circuit means which whenenergized renders said cathode emissive at a standby value, timing meanshaving a periodically actuated switch interlocked with said circuitmeans whereby said cathode is periodically rendered emissive at arelatively high value for short intervals and means responsive to thestriking of an are between said cathode and anode for rendering saidtiming means ineffective to render the cathode highly emissive, andmeans controlled by the arc responsive means for influencing saidcircuit means to set the cathode emission at a steady value intermediatesaid standby value and said relatively high value.

2. In a control apparatus, in combination, means forming a cathode,means forming an anode, said cathode and anode being adapted to have anarc struck therebetween,

means including an electrical circuit for supplying ,power' to saidcathode for rendering it emissive, means comprising a periodic timerincluding a switch which is periodically opened and closed and meansincluding said switch whereby the power supplied to the cathode isperiodically.-

6 action of the timer in changing the emissivity of the cathode uponactuation of the switch.

3. In a system of the character described comprising a cathode and ananode adapted to have an arc struck therebetween and having one powersupply for the cathode and a second power supply for sustaining the arc,in combination, means including a circuit for supplying power to thecathode for rendering it emissive, means comprising a periodic timercontrolling said circuit and having a switch interlocked with saidcircuit to periodically raise the emissivity of the cathode to arelatively high value, means responsive to the striking of an arccomprising a switch operative to be actuated upon striking of the arc,and means interlocking said last switch with said timer and arranged todiscontinue the action of the timer to increase the emissivity of thecathode, the cathode power supply being at a relatively lower valueduring the periods of discontinuance of the timer action.

References Cited in the file of this patent UNITED STATES PATENTS1,907,508 Coolidge May 9, 1933 2,112,718 Somers Mar. 29, 1938 2,151,602Kearsley Mar. 21, 1939 2,160,605 Suits May 30, 1939 2,236,195 McKessonMar. 25, 1941 2,285,450 McCarthy June 9, 1942 2,375,967 Vaughan May 15,1945

